1. Field of the Invention
Embodiments of the present invention relate to a drive train suitable for being connected to an AC electrical grid and comprising: an electric machine consisting of a rotor and a stator with the stator being electrically connected to the AC grid and having a stator frequency, a bidirectional conversion system of an AC electric current into another AC electric current, suitable for being connected between the AC grid and the rotor of the electric machine, with the bidirectional conversion system comprising: one bidirectional alternating current converter into direct current suitable for being connected to the alternating current grid, one bidirectional converter of direct current into alternating current connected between the AC-DC converter and the rotor, with the direct-alternating converter consisting of electronically controllable switches suitable for converting a direct voltage into an alternating voltage and of a connecting terminal with the rotor for each phase of the alternating voltage, and one control device of the switches of the direct-alternating converter being subject to a control law.
Embodiments of the present invention also relate to an electrical energy generation installation for an AC electrical grid, with the installation comprising a turbine and a similar drive train connected to the turbine, on the one hand, and suitable for being connected to the AC grid, on the other hand. Embodiments of the present invention are suitable for a wind turbine, or for a hydroelectric installation.
2. Description of Related Art
A drive train of the aforesaid type is known from the document “Grid connection of doubly fed induction generator wind turbines: a survey” of Martinez de Alegria & others. The drive train is connected to a three-phase grid and consists of a doubly-fed electric machine, with the stator of the machine being connected to the three-phase grid and the rotor being powered by means of a three-phase-three-phase converter connected to the three-phase grid. The converter consists of a rectifier connected to the three-phase grid and of an inverted rectifier connected between the rectifier and the rotor of the machine. In the event of a fault, such as a short circuit occurring on the three-phase grid, the drive train is obliged to conform to certain rules imposed by the electric grid (from English grid code), such as for example the German electrical grid or also the English electrical grid, in order to allow the grid to rectify this fault (from English Fault Ride Through-FRT).
This type of short circuit on the electrical grid involves a significant increase in the induced voltage at the rotor of the electric machine. The above-mentioned document then envisages different solutions in order to avoid a deterioration of the drive train, and in particular of the converter, as a result of this increase in induced voltage whilst still observing the rules of electrical grid in question.
Multiple solutions are envisaged such as the addition of a safety device against over-voltages and/or excess currents connected between the output terminals of the inverted rectifier on the AC side or even the addition of thyristors connected in anti-parallel between the electrical grid and the stator of the machine for each phase of alternating current. A solution is the addition of the safety device against over-voltages and/or excess currents combined with a rotor flow control system via a particular command from the converter that is connected between the rotor and the grid.
In any case, these different solutions necessitate the adding to the doubly-fed electric machine of an additional safety device, such as a device for protection against over-voltages and/or excess currents or thyristors connected in anti-parallel between the electrical grid and the stator of the machine.